1. Technical Field
The present invention relates to systems and methods for cylinder air charge estimation used in controlling an internal combustion engine.
2. Background Art
Precise air/fuel ratio control is an important factor in reducing feed gas emissions, increasing fuel economy, and improving driveability. Current internal combustion engine designs use various temperature, pressure, and flow sensors in an attempt to precisely control the amount of air and fuel, and thus the air/fuel ratio, for each cylinder firing event. However, due to various sensor limitations such-as response time and being located away from the combustion chamber of the cylinder, it is difficult to precisely measure and coordinate or synchronize the air and fuel quantities which are actually combusted in the cylinder. Acceptable control strategies have been developed to compensate for various sensor limitations under steady-state operating conditions. Effort is now being focused on improving these strategies to provide more accurate air/fuel ratio control during transient as well as steady-state operating conditions.
Electronically controlled throttle valve actuators have been used to improve transient air/fuel ratio control by providing increased control authority over airflow. By eliminating the mechanical linkage between an accelerator pedal and the throttle valve, the engine controller can control throttle valve position to deliver the proper airflow for current driver demand and operating conditions.
Airflow is typically measured using a mass airflow (MAF) sensor positioned upstream of the throttle valve. Intake air travels past the MAF sensor, through the throttle valve and into the intake manifold where it is distributed to a bank of cylinders. Intake air enters a cylinder upon the opening of one or more intake valves. Fuel may be mixed with the intake air prior to entering the cylinder or within the cylinder for direct injection applications. The response characteristics of current MAF sensors coupled with the delay time associated with throttle valve positioning, transit time of the air mass between the MAF sensor and the cylinder, and response time of the fuel injector, make it difficult to accurately determine the precise quantity of air and fuel in the cylinder.
Various prior art approaches have attempted to improve air/fuel ratio control and compensate for one or more of the above factors. For example, one approach attempts to synchronize throttle valve positioning commands and fuel injection commands in the crank-angle domain so that throttle valve movement is prohibited after air flow measurement. Another approach delays throttle valve movement to allow time for the fuel system to react. One strategy which provides a future estimate of cylinder air charge linearly extrapolates a current airflow measurement for a future fuel injection event. However, this method assumes air charge changes at a constant rate and does not compensate for airflow sensor filtering effects which lead to an attenuated and delayed response.
It is an object of the present invention to improve air/fuel ratio control using feed-forward observer-based controls to provide an estimate for future cylinder air charge during a future fuel injection event.
In carrying out the above object and other objects, features, and advantages of the present invention, a system and method for controlling an internal combustion engine having an electronically controlled airflow actuator, such as a throttle valve or intake/exhaust valves, include predicting current and future positions of the airflow actuator using an actuator model with the future position preferably corresponding to a subsequent injection of fuel into the cylinder, generating a delta mass air flow prediction based on current and future air flow estimates, and estimating air charge in the cylinder for the subsequent injection of fuel based on the delta mass air flow prediction. In one embodiment, the delta mass air flow prediction is used as a feed-forward term which is added to the current mass airflow sensor reading with the result processed by an intake manifold filling model to provide a future estimate of in-cylinder air charge. In another embodiment, the current and future mass airflow estimates are processed by an intake manifold filling model to produce corresponding cylinder air charge estimates. The difference of the estimates is then used as a feed-forward term that is combined with the air charge calculated using the intake manifold filling model with the sensed mass airflow as an input.
The present invention includes a number of advantages relative to prior art control strategies. For example, improved air charge estimation using the present invention allows more precise air/fuel ratio control. A priori knowledge of airflow control actuator position provides advanced notice via system modeling of how the cylinder air charge will be effected and allows improved air/fuel ratio control, especially during transient conditions. This may lead to reduced feedgas emissions and a corresponding reduction in the necessary size of the catalyst. A faster control system response without inducing instability or noise using the present invention may also result in improved driveability.